The measurement of soil moisture at various depths in the soil is an increasingly important activity in modern agriculture. Crop yields are improved, water and fertilizer are conserved, and diseases are prevented when irrigation water is applied in a manner that avoids stress to a crop. One cause of crop stress is allowing the soil to become too dry. Other stress factors are root asphyxiation and excessive leeching of nutrients due to over-watering. Soil moisture measurements taken at various soil depths provide the necessary feedback to facilitate precise scheduling of irrigation.
A variety of sensors have been developed to detect moisture in various media. These include conductivity sensors and sensors of bulk dielectric constant. Methods used for measuring the dielectric constant include time domain reflectometry or transmissometry, frequency domain reflectometry (FDR), capacitance probe (CP), and ground-penetrating radar (GPR). These methods exploit the high dielectric constant of water relative to that of the medium being measured in order to extrapolate the moisture content of the medium.
Soil permittivity measurements have become the standard means of deriving soil moisture content. Time Domain Reflectometer (TDR) and Time Domain Transmissometer (TDT) devices have been developed for use in soil moisture studies. Recent advances using low cost digital signal processing in conjunction with these devices has yielded high accuracy and stability even in the presence of temperature variations and moderate concentrations of salts and other ionic material in the soil. Reference is made to U.S. Pat. Nos. 6,657,443 and 6,831,468. Time Domain Transmissometers are well suited for permanent installation where continuous moisture monitoring is needed for closed-loop irrigation control. They are not well suited for probing-type measurements because they cannot be installed in the soil without excavation. This hindrance arises from the fact that the transmission line attached to the device is in the form of a loop and is not easily inserted into the soil. In contrast, Time Domain Reflectometers typically have an open-ended transmission line. The ends of the two-wire line can be sharpened such that insertion into the soil becomes practical without excavation. Thus probing-type measurements can readily be taken with a TDR probe but only at the depth that the fixed transmission line will allow.
Crop growers need soil moisture data throughout the root zone and also in the subsoil. Devices have been developed that are inserted into a PVC pipe-lined bore hole which measure the capacitance of the electric circuit formed by the plastic pipe wall thickness and a volume of soil outside the pipe. Cylindrically-shaped capacitor plates inside the pipe define the capacitor geometry. These plates can be moved up and down inside the pipe to take capacitance readings at various depths. The capacitance readings can then be related to moisture content. The electric field in the soil outside the pipe is distorted by conductive losses in the soil causing the output reading to be dependent on soil chemistry. Hence the devices are used in a relative mode, that is, they must be calibrated for the soil in which they are inserted and the readings must be referenced to that calibration point. If the soil chemistry changes due to salts or fertilizer content of the soil, then the calibration becomes invalid. The accuracy of such devices is also inferior to the TDR and TDT devices discussed above.
Current electronic methods used for monitoring soil moisture are subject to errors caused by compaction, electrical conductivity and temperature of the soil. Sensing devices must be calibrated for the soil and the readings must be interpreted by someone trained in the use of the specific sensing device. The improved sensor of the presently described invention reports absolute soil moisture at any desired soil depth. This improved sensor does not need to be calibrated for the soil. Its readings are stable with changing soil temperatures, electrical conductivity and compaction. The resulting data may be easily and reliably used by the crop grower without need of a consultant to interpret the readings. The data is of sufficient accuracy and stability that automatic, closed-loop irrigation scheduling may become the standard practice among early-adopting growers.